ACRO mode is where the sticks control the speed at which the quadcopter pitches, rolls, or yaws. When the sticks are released and come back to default position the quadcopter will maintain its angle.
Ardupilot.com definition "Acro mode (Rate mode) uses the RC sticks to control the angular velocity of the copter. Release the sticks and the vehicle will maintain its current attitude and will not return to level. Acro mode is useful for aerobatics such as flips or rolls, or FPV when smooth and fast control is desired."